A number of publicly available communication systems use frequency pairing to facilitate communication between transceivers. An excellent example is a cellular communication system which uses one frequency for transmitting and another for receiving. In some communications systems the frequencies are paired, meaning that the frequency difference is constant between transmit and receive frequencies, and different pairs may be used by different equipment within the system. Thus, communication devices for use in such systems must generate one frequency for transmitting signals, and another for receiving signals.
In some systems it is necessary to have two frequency generation circuits, one for transmitting and one for receiving for both carrier and intermediate frequencies. This is because some communication systems providing full duplex service transmit and receive at the same time. In many digital systems, however, due to data and voice compression techniques, it is not necessary for a communication device to transmit and receive at the same time. A popular technique in digital wireless communication systems is to define channels by both frequency and time slots, and is well known as time division multiplexing (TDM), or time division multiple access (TDMA). In wireless TDM systems, such as, for example, digital cellular telephony, a first carrier frequency is used to transmit information from a base station to multiple mobile stations, each assigned to a particular, periodic time slot. Similarly, the mobile stations take turns transmitting information to the base station at different, periodic time slots. Typically the transmit and receive time slots are staggered so that transmitting and receiving do not occur at the same time at the mobile station.
In theory this means in a TDMA system it is not necessary to have two dedicated frequency generation circuits. A single circuit that can switch between receive and transmit frequencies efficiently could be used. This would serve other goals as well. It is well known that in most markets, smaller and lighter communication devices are more desirable by consumers, and having only one circuit would help minimize size and weight. In practice, however, there are significant challenges to overcome to achieve the goal of a single frequency generation circuit for both transmit and receive frequencies. For example, the transient response of the frequency generation circuit in changing from one frequency to another must be sufficient to allow stable switching in the interim between receiving and transmitting. A circuit with acceptable transient response is realizable with conventional technology, but at a cost premium. Additionally, frequency generation circuits have a practical limit as to their frequency range. A typical frequency generation circuit uses a varactor as an adjustable tuning element, and a control voltage is used to select a particular operating frequency. However, the range of the available control voltage may be limited, as in portable battery powered communication devices, and the varactor itself has a practical limit as to the amount of tuning range it can provide. If the transmit and receive frequencies are sufficiently different, a single oscillator circuit might not be able provide sufficient range, and thus two oscillators are often used, particularly for intermediate frequency generation. As it is desirable to reduce cost at the same time as reducing size and weight, such a circuit is not preferred. Therefore there is a need for a frequency generation circuit that has the advantages of reduced size, and fewer parts, and lower operating power compared to two completely separate frequency generation circuits, and avoids the higher costs associated with more sophisticated frequency generation circuits having the ability to switch between transmit and receive frequencies at the desired rate.